The aramid fiber Nomex has several interesting and indeed unique properties that make it highly desirable for specific applications. As an example, the fiber is inherently flame retardant, and thus lends itself to applications in which such fire retardant properties are desirable. Aramid fibers are extremely hard to dye, due, in part, to the crystallinity of the fiber. Solution-dyed aramid fibers are available but only in limited colors as selected by the fiber manufacturer. Frequently dye carriers such as acetophenone have been used to effectively dye such fibers. However, these carriers may themselves be flammable or detract from the otherwise good inherently flame retardant properties of the fiber. In addition, acetophenone dyed products have a rather characteristic odor that is not well accepted. Dyes introduced into the fiber and/or topical finishes and chemicals applied to the fiber affect the fire retardant characteristics of the final product.
Observations with respect to the fire retardant properties, or lack thereof, for Nomex fibers are similar to those noted in the dyeing and finishing of glass fabrics. While glass fibers will not themselves support combustion, the pigments, dyes or other finishes applied to glass fabrics will burn. Glass fabric forms a convenient matrix to support the pigments, dyes or finish, and this matrix provides a relatively large surface area and allows flame to propagate. A similar situation occurs with Nomex fabrics and thus, as with glass fabrics, it is sometimes necessary to add an auxiliary fire retardant chemical to the finish to improve the overall fire retardant characteristics of the final Nomex product.
It is therefore an object of this invention to both dye and fire retard aramid fibers, specifically Nomex, in a single step and to provide a product that has fire retardant qualities that are at least as good as, if not better than, undyed, greige Nomex fabric. The invention includes the application of a fire retardant material or fire retardant system together with a disperse dye or an acid dye (anionic dye) to a aramid fiber in the form of a staple, tow, or yarn; woven, non-woven, circular knitted, or tricot knitted fabrics; crimped, texturized, flocked, or tufted textiles; but preferably in the form of a woven fabric. The acid or disperse dye may be applied to the fibers using any convenient process; however (1) a pad/thermosol process, (2) a print paste process, or (3) immersion of the fibers into a neat, heated solution of fire retardant plus dyestuff gives the best results.
Candidate fibers for the dyeing and fire retardant process of this invention are known generally as aromatic polyamides or "aramids". This class includes a wide variety of polymers as disclosed in U.S. Pat. No. 4,324,706, the disclosure of which is incorporated by reference. Experience indicates that for some reason not fully understood, not all types of aramid fibers can be reproducibly treated by this process; those fibers that are not suited do not allow the disperse or acid dye to enter the fiber and are only surface stained. Thus, a fiber amenable to the process of this invention is selected from the class of aramids suitable for dyeing and fire retarding. Such fibers are referred to here as treatable, i.e., dyeable and fire-retardant treatable aramids, the best known being the polymer known chemically as poly(m-phenyleneisophthalamide), i.e., the meta isomer which is the poly condensation product of m-phenylenediamine and isophthalic acid. Below is a listing of fibers now commercially available identified by fiber name (usually a trademark) and producer:
______________________________________ Fiber Name Producer ______________________________________ Nomex DuPont Apyeil Unitika (5207) Apyeil-A Unitika (6007) Conex Teijin ______________________________________
Selection of a suitable aramid amenable to the continuous dyeing and fire retarding process of this invention can be conveniently made by subjecting a fiber sample to an abbreviated test to determine fiber acceptability. Experience indicates that fibers of the para isomer, poly(p-phenyleneterephthalamide), represented commercially by duPont's Kevlar and Enka-Glanzstoff's Arenka, as well as Rhone-Poulenc's Kermel and polybenzimidazole (PBI), are merely stained or changed in color but are not dyed by the process of this invention. Accordingly, as used in the text of this application and in the claims that follow, the expressions "aramid" and "aromatic polyamide fiber", when pertaining to the novel process of this invention, will signify the meta isomer. Blends of poly(m-phenyleneisophthalamide) fibers with other fibers, including fibers of the para isomer, may be subjected to the dyeing process in which case only the meta isomer fibers will be dyed.
The fire retardant materials used in the process of this invention do not degrade and successfully withstand heat treatment at temperatures over 300.degree. F., and are typically liquid at such temperatures. Among the types of materials that may be employed, preferred are the cyclic phosphonate esters described, for instance, in one or more of U.S. Pat. Nos. 3,894,386, 3,149,476, 3,991,019 and 3,511,857.
The fire retardant materials used in accordance with the present invention are thermally stable cyclic phosphonate esters prepared by reacting alkyl-halogen-free esters with a bicyclic phosphite. As a class these cyclic phosphonate esters are represented by one of the formulas: ##STR1## where a is 0 or 1; b is 0, 1 or 2, c is 1, 2 or 3 and a+b+c is 3; R and R' are the same or different and are alkyl (C.sub.1 -C.sub.8), phenyl, halophenyl, hydroxyphenyl, tolyl, xylyl, benzyl, phenethyl, hydroxyethyl, phenoxyethyl, or dibromophenoxymethyl; R.sup.2 is alkyl (C-C.sub.4 ); and R.sup.3 is lower alkyl (C.sub.1 -C.sub.4) or hydroxyalkyl (C.sub.1 -C.sub.4 ) or ##STR2## where d is 0, 1 or 2; e is 1, 2or 3; R.sup.2 is alkyl (C.sub.1 -C.sub.4); R.sup.3 is lower alkyl (C.sub.1 -C.sub.4) or hydroxyalkyl phenyl, halophenyl, hydroxyphenyl, hydroxyethyl, phenoxyethyl, dibromophenoxyethyl, tolyl, xylyl benzyl, or phenethyl; and R.sup.5 is monovalent alkyl (C.sub.1 -C.sub.6), chlorophenyl, bromophenyl, dibromophenyl, tribromophenyl, hydroxyphenyl, naphthyl, tolyl, xylyl, benzyl, or phenethyl; divalent alkylene (C.sub.1 -C.sub.6), ylene, o-phenylene, m-phenylene, p-phenylene, tetrachlorophenylene (o, m, or p), or tetrabromophenylene (o, m, or p); or trivalent phenenyl.
The preferred compounds are represented by the formula: ##STR3## in which x is 0 or 1, usually a 50:50 mixture of the mono and di-esters. The preparation of these cyclic phosphonate esters and their use as flame retardants are described in U.S. Pat. Nos.3,789,091 and 3,849,368, the disclosures of which are hereby incorporated by reference.
The 50:50 mixture of these esters is available as Antiblaze 19 (sometimes AB 19 herein) from Albright & Wilson, Inc., of Richmond, Va. Also available is Antiblaze 19T, a low viscosity grade flame retardant containing 93% active ingredient and formulated especially for textile treating requirements. As described by the supplier, Antiblaze 19 has a flash point of 464.degree. F. (240.degree. C.), and 19T a flash point of 459.degree. F. (237.degree. C.) by the Cleveland open cup method; both are suited for application as high temperatures.
An essential part of the present invention is heating the fibers in the presence of both the dyestuff (disperse or acidic) plus the fire retardant liquid. Treatment temperatures in the range of 300-600.degree. F. are contemplated. However, higher and lower temperatures may be employed depending upon the specific heat characteristics of the aromatic polyamide fiber being treated, heat tolerance of the dyestuff itself, and the nature of the fire retardant liquid. Heating is generally in the range of about 350.degree. F. to about 390.degree. F. and for a period of time sufficient to impart the desired fire retardant characteristics to the fiber as well as to introduce a sufficient quantity of dyestuff into the fiber. Exposure times range from periods as short as 10 seconds up to 2 minutes or longer, depending upon the processing conditions and the equipment employed.
Dyestuff-containing compositions and fire-retardant treatments are detailed below.
The pad/thermosol process. A solution of "neat" (undiluted) fire retardant containing the desired quantity of disperse or acid dye is padded onto the fibers at ambient temperatures, then heat treated in order to "fix" the dyestuff to the fiber and provide the required fire-retardant treatment. Following this, the fibers are washed with an aqueous detergent (scoured) then given a rinse in a halogenated hydrocarbon, for instance perchloroethylene.
Print pastes. A paste of disperse dyestuff is made with the liquid fire-retardant material, and this water-free paste is then applied to the aramid fabrics to be treated either in a uniform manner, such as with a doctor knife, nip roll or the like, or in a predetermined pattern on a printing machine. Heat is applied in order to fix the dyestuff to the fiber and accomplish the required fire retardant treatment, and this is followed by an aqueous detergent scour and a rinse in a halogenated hydrocarbon.
Immersion in hot fluid. Successful dyeing and fire retardant treatment may be accomplished by immersing the fibers, typically in fabric form, into a bath containing the fire retardant material in which the requisite quantity of disperse or acid dye has been dissolved. When Antiblaze 19 is used as the fire retardant liquid, the dyestuff solution is maintained at a temperature in the range of about 350 to 380.degree. F., and the fibers are exposed to the heated solution for various periods of time ranging from as little as 15 seconds up to about 2 minutes. This immersion is followed by an aqueous detergent scour and then rinsing with a halogenated hydrocarbon.
While not wishing to be bound by any theory or mode of operation, it would appear that a suitable fire retardant acts both as a solvent or a vehicle for dyeing the fiber and causing the fiber itself to swell, thus allowing the disperse or acid dye to enter into the fibers. In addition, it appears that the dyeing mechanism is an equilibrium condition between the fiber and the fire-retardant fluid--the greater the solubility of the dye in the fire-retardant liquid, the less the "solubility" of the dye in the fiber. The dye yield thus depends upon solubility of the dye in the fire retardant fluid.
The process of the present invention makes it possible to both dye and improve the fire retardant characteristics of aramid fibers using either acid dyes or disperse dyes with the minimum number of steps, at a rapid rate of treatment and on existing equipment. The dyes can be applied by a pad/thermosol process, immersion in the hot fire-retardant fluid containing the dye, or by incorporating the fire-retardant fluid into a print paste and printing the fabric. The fire-retardant fluid may also contain an organic solvent swelling agent to assist in swelling the aramid fiber and thus facilitate introduction of the dyestuff and flame-retardant into the fiber. Such swelling agents include N-methylpyrrolidone, dimethylsulfoxide (DMSO) and dimethylacetamide.